In recent years, light emitting diodes (LEDs) capable of emitting light with high luminance and high illuminating efficiency have been developed. In comparison with a common incandescent light, a LED has lower power consumption, long service life, and quick response speed. With the maturity of the LED technology, LEDs will replace all conventional lighting facilities. Until now, LEDs are widely used in many aspects of daily lives, such as automobile lighting devices, handheld lighting devices, backlight sources for LCD panels, traffic lights, indicator board displays, and the like.
Generally, the LED can be considered as a DC load. When an electronic device (e.g. a LCD panel) having multiple LED strings is operated, the currents passing through all LED strings shall be identical for a purpose of obtaining uniform brightness. Due to different inherent characteristics of these LED strings, the currents passing these LED strings are not identical and the brightness is usually not uniform. Therefore, the use life of individual LED string is shortened or even the whole electronic device has a breakdown.
For obtaining uniform brightness of multiple LED strings, several current sharing techniques have been disclosed. For example, as shown in FIG. 1, U.S. Pat. No. 6,621,235 disclosed a current sharing supply circuit for driving multiple LED strings. The current sharing supply circuit of FIG. 1 principally includes a linear regulator 11, a low-pass filter 12 and multiple current mirrors M1˜Mn. A constant reference current Iref is inputted into a first terminal of the linear regulator 11. The linear regulator 11 is controlled with the constant reference current Iref and thus an output voltage is generated and transmitted to the low-pass filter 12. The output voltage is filtered by the low-pass filter 12 and then transmitted to the gates of the current mirrors M1˜Mn. As a consequence, these current mirrors M1˜Mn, outputs identical currents. In other words, the LED strings linked to the current mirrors M1˜Mn have the same current and brightness.
The conventional current sharing supply circuit for driving multiple LED strings, however, still has some drawbacks. For example, since the linear regulator and the current mirrors are employed, the conventional current sharing supply circuit has high power loss but low operating efficiency. In addition, since more components are used, the conventional current sharing supply circuit is very complicated.
There is a need of providing a current-sharing transformer so as to obviate the drawbacks encountered from the prior art.